Dataset Explorer "Gross Volume" vs "Gross Volume Modelled"

I seem to be getting some unexpected results out of Dataset Explorer. the attached extract shows the read-out from a beam spanning between two others (highlighted in pink). The beam was modelled with Structural Snaps switched on...

I don't understand what the figures for "Feature Volume", "Gross Volume" (=0???), "Net Volume" (=negative number) are supposed to represent in this instance.

Six rows below, in the same table, is the same beam modelled with Structural Snaps switched off. this now gives a different set of figures. I assume that the difference is that the structural snaps beam has a "Feature" cut off at each end - hence the "Feature Volume". But why is the "Gross Volume"  zero, in all cases?

Presumably the "Gross Volume (modelled)" and "Net Volume (modelled)" are the accurate values. Why confuse things with the other columns? Please can someone explain what they show.

Mark

ABD (SS4) v 08.11.09.622 - ANZ DataSet